This invention relates to a receiving apparatus and a receiving method used for, for example, portable telephones, and a communication system formed using the portable telephones.
In a portable telephone of a communication system using the spread spectrum transmission, while the portable telephone is in a so-called waiting status during which the telephone is detecting an incoming call the control signal transmitted from the base station is received and is correctly synchronized with a demodulating signal in the telephone so as to be demodulated.
This synchronization correction denotes a processing for synchronizing the generation timing of each PN code (pseudo random noise code) generated in the portable telephone with the generation timing of each PN code used for spread-modulation of the transmitted control signal in order to correctly demodulate the control signal which is spread-modulated by using the PN code.
In this case, the portable telephone detects a deviation between frequencies of both PN codes using the PN code generated according to the clock signal from the variable frequency oscillator of the telephone and the received control signal. According to the detected deviation between those frequencies, the oscillating frequency of the variable frequency oscillator of the telephone is corrected so that the generation timing of the PN code generated in the telephone can be synchronized properly with the generation timing of the PN code used for spread-modulation of the received control signal.
After this, the PN code generated in the telephone and corrected as described above is used to demodulate the received control signal into its original form before spread-modulation. The demodulated control signal is then used to get transmitted information and decide whether or not there is any call coming in to the telephone.
However, in such a portable telephone of a communication system to which the spread spectrum transmission is applied as described above, when the telephone is in the waiting status, an intermittent receiving processing is usually executed as shown in FIG. 1. This is because the telephone in the waiting status is only expected to detect call properly, so that there is no need to execute a receiving processing continuously.
Power consumption of such a portable telephone in the waiting status can thus be saved by executing a receiving processing in each receiving period TA and by executing no receiving processing in each non-receiving period TNA as shown in FIG. 1.
In such a portable telephone, each receiving cycle TS shown in FIG. 1 should preferably be set longer for consumption power saving. In other words, if the non-receiving period TNA is set longer while the number of receiving processings is reduced, the power consumption can be reduced.
In such a portable telephone, however, the accuracy of the receiving cycle TS depends on the accuracy of the variable frequency oscillator that decides the PN code generation timing. If the non-receiving period TNA is set longer while the variable frequency oscillator accuracy is not so high, then the synchronization between the above timings will be lost in a non-receiving period TNA and this will make it impossible to get the control signal in the next intermittent receiving processing period.
One of the measures for setting each non-receiving period longer is to use a high precision frequency oscillator. However, such a high precision frequency oscillator is expensive and rises the price of the portable telephone.
Accordingly, an ordinary portable telephone has used a low-price oscillator to avoid an incease of the price. The receiving cycle TS had thus to be set short. And, such a portable telephone requires frequent battery replacement or battery charging, which sometimes makes the telephone very inconvenient.
Under such circumstances, the object of this invention is to provide a receiving apparatus and a receiving method that can keep a proper synchronization accuracy and reduce power consumption while the telephone is waiting for a call without increasing the price of the portable telephone.